1. Field of the Invention
This invention relates to multilayered plastic molded objects that consist predominantly of an aromatic polycarbonate, and that are provided with a polymethacrylate plastic film containing a UV absorber on their surface as protection against the action of UV radiation.
2. Discussion of the Background
Polycarbonate plastics have only limited weather resistance. They are particularly damaged by UV radiation, which is manifested by originally glossy surfaces becoming dull The dulling is the visible symptom of macromolecular degradation caused by the UV radiation with the formation of a number of extremely fine hairline cracks beginning in the plastic surface. UV absorbers introduced into the plastic are able to protect the interior of the plastic object, for example a plastic sheet made of polycarbonate, but not the surface. To protect the surface, a varnish film of polymethyl methacrylate which also contains a UV absorber is applied from a solution of these protective film components according to DE 1,694,273 (U.S. Pat. No. 3,582,398).
U.S. Pat. No. 4,666,779 also describes UV-stabilized objects made of thermoplastic substrates, particularly those made of polycarbonates with a protective film of a copolymer that consists essentially of methyl methacrylate units and 0.5 to about 10% of the number of monomer units of other methacrylate units, specifically ethyl methacrylate, n-butyl methacrylate, iso-butyl methacrylate, or mixtures of these, and also 0 to 5% of the number of monomer units of other .alpha.,.beta.-unsaturated monomer units. The polymethacrylate protective film is coated on the substrate as a solution, with the low molecular weight UV stabilizers needed for UV stabilization being applied with the polymer-solvent system.
According to DE-A 28 32 676, a polymethyl methacrylate film with an adequate content of a UV absorber is applied during the manufacture of a polycarbonate plastic sheet by coextrusion. According to DE-A 32 44 953 (EP-A 110 238), an improved composite object is obtained by applying still another topcoat of polymethyl methacrylate or (meth)acrylate copolymers essentially containing little UV absorber to the plastic film containing the UV absorber, which can be polymethyl methacrylate or polycarbonate, by a multiple-component extrusion process.
In all cases described in the art, polymethacrylate plastics, particularly polymethyl methacrylate or copolymers of methyl methacrylate and lesser amounts of C.sub.2 -C.sub.4 -alkyl methacrylates are preferably used for the topcoat containing UV absorbers, because they have high weathering resistance and their surfaces are damaged less by UV radiation than those polycarbonates.
The drawback of known polymethacrylate topcoats containing UV absorbers for polycarbonate plastics is that they adhere inadequately to the polycarbonate being covered. This characteristic can lead to the topcoat detaching at least partially from the polycarbonate core, whereby its UV protection is lost. The detachment of such topcoats is promoted by both thermal and mechanical stresses, for example during the bending of multilayered plastic molded objects. The loss of UV protection leads to the molecular degradation of the polycarbonate plastic object and thus to the loss, or at least to the reduction of its mechanical properties.
In addition, plastic objects are also disclosed by U.S. Pat. No. 4,576,870 and EP-A 0 203 487 in which the UV absorbing protective film is a copolymer of essentially (meth)acrylic ester monomers with monomers containing UV-protective groups. These protective films are generally even more brittle than the protective films containing low molecular weight UV absorbers described above.